Method of forming powder metal components having surface densification

ABSTRACT

A method for producing a powder metal article having a three dimensional shape and having at least one densified surface comprising: a) providing a blend of powdered metals; b) compacting said blend to form a pre-form having a general shape of said article; c) sintering said pre-form; d) densifying at least one cylindrical surface region of said pre-form; and, e) forming said pre-form to a final density and into the three dimensional shape of said article.

FIELD OF THE INVENTION

The present invention relates to methods of manufacturing powder metalcomponents. More specifically, the invention provides a method ofdensifying the core and surface of a powder metal component to achieve aproduct having an evenly densified surface.

BACKGROUND OF THE INVENTION

Powder metal (PM) technology is a lower cost alternative for producingcomponents that could be made from wrought metal. The use of PMcomponents is precluded in many applications because of inferiormechanical strength caused by residual porosity. Therefore, in themanufacture of PM articles, the achievement of high density, close tothat of wrought steel (generally assumed to be approximately 7.86 g/cc),is of significant importance since the strength and durability of a PMarticle is directly related to its density. Typically, the basic stepsinvolved in the manufacture of a powder metal component are: a) blendingthe desired metal powders; b) compacting the powder to the desiredshape; c) sintering the compact; and d) forming the sintered compact tothe desired final shape. The final step is used to impart the requireddimensional features of the article. Following the forming step, it isalso common to perform a heat treatment on the finished article toimpart, where desired, certain mechanical properties as known in theart.

The final density of a PM article is dependent on the characteristics ofthe powders in the blend, sintering conditions, and the compressiveforces applied to the article primarily during the compaction andforming steps. It is common in known methods to compact a powder blendto a moderate initial density, approximately 7.0 g/cc, and furtherdensifying the compact during subsequent forming steps. Variouscompositions of metal powder blends are known in the art as are methodsof compaction and sintering. Examples of known blends and methods aretaught in U.S. Pat. No. 5,476,632 (incorporated herein by reference).

For manufacturing articles having a bearing surface and the like, it isknown to increase the surface density at the desired locations toprovide a densified region that is capable of withstanding the bearingforces. The prior art provides various methods for densifying surfaceportions of PM articles. For example, U.S. Pat. No. 5,540,883(incorporated herein by reference), teaches a method of densifying aselected surface portion of a sintered article by applying rollingcylinders or the like to create a bearing surface on an article. Thebearing surface, by virtue of having an increased density is bettersuited to withstand the physical stresses (i.e. rolling or slidingstresses) applied on that portion of the article. In the process taughtin the aforementioned reference, a specific section of the article canbe provided with a density that approximates the theoretical maximumvalue while the rest of the article has a density of approximately 90%to 98% of the theoretical maximum value. The reference is directed toproviding bearing surfaces or bushings, which are inherently cylindricaland do not have a complex shape. Moreover, the reference does not teachany alteration of the core density during the cylindrical surfacedensification step.

Various other surface densification methods are known in the art suchas, for example, in U.S. Pat. Nos. 6,168,754; 6,013,225; 5,884,527; and,6,110,419 (all of which are incorporated herein by reference).

For example, U.S. Pat. No. 5,884,527 teaches a method of roll forming asintered gear comprising meshing a sintered pre-form in interferencewith a rotating roll forming gear die. This method is mainly suited forsurface densifying pre-forms of specific geometries, such as gear teeth,that permit the design of a rolling die to impart the necessarycombination of line contact and relative motion between the die andpre-form.

Furthermore, U.S. Pat. No. 6,168,754 teaches a method of densifying thecontoured surface of a sintered pre-form comprising forcing said articleat ambient temperatures through a series of dies having successivelymore interference contact with the surfaces to be densified. Adisadvantage of this method is the need for multiple dies and theinherent complexity and cost.

Moreover, U.S. Pat. No. 6,013,225 teaches a method of surface densifyinga pre-form utilizing a method of selectively heating the surface of saidarticle and forcing it through a die. Inherent disadvantages of thismethod are the requirement of a separate surface heating step, decreasedtool life due to elevated die temperatures and the correspondingdetrimental effect on dimensional accuracy.

In the known methods of surface densification, the densification step istypically conducted after the forming step, when the formed article hasits final shape and is close to final core density. Moreover, the knownmethods are generally incapable of surface densifying contouredsurfaces, require unduly complex dies, and/or involve high processcosts. For example, in published U.S. application Ser. No. 10/767,014(published under number 2004/0177719), there is taught a method forsurface densification. This reference teaches a process wherein a powdermetal is compacted to the final form, sintered and then surfacedensified prior to sizing or forging. This reference stipulates that forany final surface having a complex or irregular shape, specialdensification processes are required (such as peening). Thus, the lessexpensive rolling process cannot be used for irregularly shapedarticles.

The present invention seeks to mitigate at least some of thedeficiencies in the prior art powder metal manufacturing methods.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method of producingpowder metal components with high core and surface densities and withhigh precision on contoured forms.

In another aspect, the invention provides a method of densifying asintered powder metal article by first surface densifying a cylindricalsurface on a sintered perform and subsequently forming the article tofinal core density and final shape in a closed die cavity, wherein theformed article has a compressed length of 5 to 30% less than theoriginal sintered length.

In another aspect, the invention provides a method of making a sinteredmetal article comprising: blending one or more lubricants, carbon,alloys, and iron; pressing the blend to form a compact; sintering thecompact to produce a sintered powder metal article; densifying thesurface of the article at ambient temperature by relative motion betweenthe article and a densification tool; and, forming the article in aclosed die cavity having a clearance for movement of said article so asto allow the article to assume a final shape and final density.

In another aspect, the invention provides a method of densifying asintered metal article comprising: blending one or more lubricants,graphite, iron and one or more of ferromanganese, ferromolybdenum andferrochromium; pressing the blend to form a compact; sintering thecompact at a temperature of at least 1250° C.; surface densifying atleast one cylindrical surface of the sintered article by rollerburnishing; forming said article at between 600 and 1300 MPa in a closedcavity so as to produce a final part with core a density of 90 to 98% ofthe theoretical density, a compressed length of 5 to 30% less than thesintered length and contoured densified surfaces.

In another aspect, the invention provides a method of making a sinteredmetal article by blending one or more lubricants, carbon, and ironpowder pre-alloyed with Mn, Mo, Cr, Ni, etc; pressing the mixture, orblend, to produce a compact; sintering the compact at a temperature ofat least 1100° C.; surface densifying at least one cylindrical surfaceof the sintered article by roller burnishing; forming the article to afinal core density and shape in a closed die cavity; wherein the formedarticle has a compressed length of 5 to 30% less than the originalsintered length.

In another aspect, the invention provides a method of making a sinteredmetal article by: blending one or more lubricants, carbon, and elementalor substantially pure iron and one or more of Mn, Mo, Ni, Cu, etc inelemental form; pressing the mixture, or blend, to produce a compact;sintering the compact at a temperature of at least 1100° C.; surfacedensifying at least one cylindrical surface of the sintered article byroller burnishing; and subsequently forming the article to a final coredensity and shape in a closed die cavity, wherein the formed article hasa compressed length of 5 to 30% less than the original sintered length.

In a further aspect, the invention provides a method of producingoverrunning clutches or the like with high core density and densifiedcontact surfaces.

Thus, in one aspect, the present invention provides a method forproducing a powder metal article having a three dimensional shape andhaving at least one densified surface region, the method comprising:

a) providing a blend of powdered metals;

b) compacting the blend to form a pre-form having a general shape of thearticle, the preform being generally cylindrically shaped at a regioncorresponding to the at least one densified surface region;

c) sintering the pre-form;

d) densifying the at least one surface region of the pre-form; and,

e) forming the pre-form to a desired final density and into a desiredthree dimensional shape of the article.

In another aspect, the present invention provides a method for producinga powder metal article having a three dimensional shape and having atleast one densified surface region, the method comprising:

a) providing a blend of powdered metals;

b) compacting the blend to form a pre-form having a general shape of thearticle, the pre-form having a density of between 70% to 90% of thetheoretical maximum density and being generally cylindrically shaped ata region corresponding to the at least one densified surface region;

c) sintering the pre-form;

d) densifying the at least one surface region of the pre-form to atleast 90% of the theoretical maximum density; and,

e) forming the pre-form to a desired final density and into a desiredthree dimensional shape of the article.

In another aspect, the invention provides a powder metal pre-form havinga general shape of a desired article, the pre-form having a density ofbetween 70% to 90% of the theoretical maximum density and beinggenerally cylindrically shaped at least one surface region.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the preferred embodiments of the inventionwill become more apparent in the following detailed description in whichreference is made to the appended drawings wherein:

FIG. 1 is a top view of a die, in an open position, with the top punchremoved, loaded with a pre-form.

FIG. 2 is a cross sectional view of the die of FIG. 1.

FIG. 3 is a top view of a die, in a closed position, with the top punchremoved, loaded with a pre-form.

FIG. 4 is a cross sectional view of the die of FIG. 3.

FIG. 5 is a graph comparing the sub-surface density gradients of asurface densified pre-form and the final C—Mn-Mo one-way clutch outerrace formed at 985 MPa.

FIG. 6 is a graph of the formed core density of a C—Mn-Mo one-way clutchouter race.

FIG. 7 is a graph of the formed closure of a C—Mn-Mo one-way clutchouter race.

FIG. 8 is a graph of the formed radial movement of a C—Mn-Mo one-wayclutch outer race.

FIG. 9 is a graph comparing the sub-surface density gradients of asurface densified pre-form and the final formed shape for a C—Mo one-wayclutch inner race formed at 925 MPa.

FIG. 10 is a graph of the formed core density of a C—Mo one-way clutchinner race.

FIG. 11 is a graph of the formed closure of a C—Mo one-way clutch innerrace.

FIG. 12 is a graph of the formed radial movement of a C—Mo one-wayclutch inner race.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, the terms indicated below will beunderstood to have the associated meanings:

“Metal powder”—a metal that is in a fine powder form. The metal may bepure (i.e. iron), pre-alloyed iron (i.e. iron alloyed with other metalssuch as, but not limited to, one or more of molybdenum, chromium ornickel), or an alloy of one or more metals (i.e. ferro manganese, ferromolydenum or ferro chromium).

“Powder metal article”—an article formed from a metal powder. The metalpowder is compressed under high pressures in a die or mould having adesired shape. The compressed article may be subjected to otherprocesses such as sintering etc. to achieve desired physical properties.

“Blend” or “Powder metal blend”—a blend of one or more metal powders andother additives such as lubricants, carbon (e.g. graphite) etc.

“Compacting”—the step of pressing a powder metal blend in a rigid dieuntil the blend assumes a desired shape and density. The compacted shapemay be the same or similar to that of the final article.

“Compact” or “pre-form”—the article resulting from compacting a powdermetal blend.

“Sintering”—a process wherein a compact is subjected to hightemperatures and selected atmospheres (e.g. a reducing atmosphere) tocause the compact to become a coherent mass by heating without melting.Sintering is normally performed on a powder metal compact to impartdesired physical properties.

“Surface densification” or “Selective densification”—the step ofdensifying a select portion of a sintered compact, usually the outersurface or a portion thereof. The densification is preferably conductedby means of rollers and the like as known in the art. However, variousapparatus for densifying surfaces will be apparent to persons skilled inthe art after reviewing the following description. The densificationstep can be conducted on the entire surface of the compact or on one ormore cylindrical portions thereof. Thus, as used herein, the term“region” or “densified surface region” will be understood to mean theentire surface or a portion thereof. The densified surface region willtypically be densified to a density of between 80% and 100%, andpreferably at least 98%, of the theoretical maximum density. Further,the depth of the densified region would be at least 0.025 mm (or 0.001inches) from the surface. In one embodiment, the densified region wouldextend to a thickness of about 1 mm (or 0.04 inches) from the surface.

“Theoretical maximum density”—refers to the density of the powder metalcompact when processed until no pores exist. In the general case, thetheoretical maximum density would be the density of wrought steel, i.e.7.86 g/cc.

“Forming”—a process of providing a sintered compact with its finalshape. This step is normally performed in a closed die or mould, whereinthe sintered compact is subjected to pressure to result in the finaldimensions and density of the final article. The forming step is knownby various terms including: sizing, coining, repressing, re-striking andpowder forging. In some cases, the sintered compact may also be heatedprior to the forming step in order to improve the malleability of thematerial.

“Annealing”—a process of treating a surface densified or formed sinteredarticle wherein the article is subjected to high temperatures in aselect atmosphere (e.g. protective atmosphere, vacuum etc.) to annealthe article to obtain an advantageous microstructure.

“Heat treatment”—a process of treating a formed sintered article whereinthe article is subjected to high temperatures, select atmospheres (e.g.protective atmosphere, vacuum, carburizing, etc.), and rapid cooling toobtain desired mechanical properties. Heat treatment methods include,but are not limited to, through-hardening, carburizing and inductionhardening which are typically followed with a tempering treatment foroptimum properties.

In one embodiment, the present invention provides a method of surfacedensifying the contoured surfaces of a sintered powder metal article,such as, for example, the cam forms of a one-way clutch, by firstsurface densifying a cylindrical surface of a lower density pre-form andforming the article to final desired shape and density in a closed diecavity. Thus, according to the invention, the surface densification stepis performed prior to the forming step. Therefore, in summary, the stepsof the invention are as follows: a) mixing one or more powder metals toform the desired blend; b) compacting the powder to create a pre-form;c) sintering the pre-form; d) performing a surface densification step onthe pre-form; and e) forming the article to the desired shape and coredensity. After the forming step, the formed article may be furthershaped and heat treated.

Formation of Powder Metal Blend

It will be appreciated by persons skilled in the art that a wide rangeof powder blends may be used in the method of the present invention. Inone embodiment, the present invention utilizes low alloy steelcompositions, where the carbon content is less than 0.7% and preferablybelow 0.3% by weight of the final sintered article.

In one embodiment of the invention, the powder compositions may compriselow cost iron powders, which are blended with calculated amounts offerro alloys, graphite and lubricant such that the final desiredcomposition is achieved following sintering and the powder blend issuited to compaction in rigid compaction dies. Examples of these powderblends are provided in U.S. Pat. No. 5,476,632 (incorporated herein byreference). The use of substantially pure iron powder admixed with ferroalloys may be preferred as such powders are relatively highlycompressible and are relatively inexpensive as compared to pre-alloyedpowders. The powder blend of the invention may comprise elemental orsubstantially pure iron powder blends, fully pre-alloyed powder blendsand partially pre-alloyed powder blends. It will be appreciated that anycomposition of powder metal blend may be used in the present invention.In one embodiment of the invention, alloys of iron, such as ferromanganese, ferro molydenum and ferro chromium may be used individually,or in combination, as required to achieve desired performancerequirements of the final article. For example, one, two, or three ferroalloys may be blended with the base iron powder. A wide range of alloyelements can be used in the process described herein, depending on thefinal product performance requirements, including: carbon, chromium,copper, manganese, molybdenum, nickel, niobium and vanadium. Alloyelements may be present either singly or in combination.

The base iron powder will generally have a particle size distribution inthe range of 10 to 350 μm. This range includes the base iron powderparticle sizes of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200,250, 300, or 350 μm and any size there-between. The alloying additionstypically will have a particle size distribution in the range of 2 to 20μm. This range includes the particle size of alloying additions of 2, 3,4, 5, 6, 7, 8, 9, 10, 15, 20 μm and any size there-between. Commerciallyavailable lubricant powder is added to the blend to facilitatecompaction. Typical lubricants include zinc stearate, stearic acid orethylene bistearamide. Various particle sizes, lubricants and otheradditives will be apparent to persons skilled in the art.

As a further alternative, the present invention may be used withpre-alloyed powder metals, some examples being molybdenum orchromium-molybdenum pre-alloys having between 0 to 1.5% of each alloywith the remainder being unavoidable impurities. With such powders,sintering can be conducted at temperatures of 1100° C. to 1150° C., oralternatively at higher temperatures greater than 1250° C. Typicalcommercial examples of prealloyed molybdenum powders are Quebec MetalPowder sold under the trademarks QMP Atomet™ 4401, and HoeganaesAncorsteel™ 85HP, both of which have approximately 0.85% by weightmolybdenum. The particle size of the pre-alloyed molybdenum powder metalis typically within the range of 45 to 250 μm. Between 0 to 0.7% carbonby weight may be added. Compacting is facilitated by the addition of thelubricants discussed previously.

Compaction of Powder Metal Blend

The compaction step is performed in the known manner using powderformulated as discussed above, whereby the blended powder is pressed ina rigid die at approximately 400 MPa to between 70 and 90% of thetheoretical maximum density. Various other pressures and end productdensities will be apparent to persons skilled in the art. Aftercompacting, the shape and dimensions of the resulting compact, orpre-form, are substantially similar to the final article with theexception of allowances for size change due subsequent operations.However, in accordance with the present invention, one or more surfaceregions to be densified are maintained in a cylindrical form. Theseareas may, in the final article, be later formed into complex,non-cylindrical shapes, as will be discussed further below. As will bemade clear in the following description, by maintaining the surfaceregions to be densified in a cylindrical form, the surface densificationof the compact is facilitated.

Sintering of Compacted Pre-Form

Next, the compacted article, or pre-form, is then sintered using methodscommonly known in the art. For example, the sintering process may beconducted in a reducing atmosphere or vacuum at a temperature in excessof 1250° C. such that oxides from both the iron and alloy additionscontained in the compact are reduced and metallurgical bonds are formedbetween contacting particles to impart strength and ductility to thesintered article. The chemical reduction process also allows for uniformdiffusion of alloying elements throughout the iron particles resultingin a homogeneous microstructure. Particularly for higher carbon contentmaterials, an isothermal hold or slow cooling treatment may also beutilised to maximize the ferrite content of said article as described,for example, in U.S. Pat. No. 5,997,805 (which is incorporated herein byreference). As will be understood, such isothermal treatment serves toimprove the malleability of the sintered article. Further, theisothermal treatment step can be included within the cooling phase ofthe sintering step (i.e. it can form a part of the sintering step) orcan be included as a separate step following sintering. In the case ofelemental powder blends and partially or fully pre-alloyed powder metal,sintering may take place at conventional sintering temperatures of 1100°to 1150° C. or at a higher temperature up to 1350° C. As known in theart, no significant densification occurs during the sintering process.As such, the density of the sintered compact will remain substantiallythe same as that of the compacted pre-form.

Surface Densification of Sintered Pre-Form

Densification of the surface, according to one aspect of the presentinvention, is generally performed using a plurality of small diameterrollers in a roller burnishing tool to cold roll the one or morecylindrical surfaces of the sintered compact. Examples of such tools andprocesses are provided in U.S. Pat. No. 5,540,883; however, variousother tools and methods known in the art may equally be used. As isknown to persons skilled in the art, the application of a rollerburnishing tool, for example, to a cylindrical surface, compresses thesurface, collapsing the pores contained therein so that the surface ofthe article has a density approaching the theoretical maximum density.

As mentioned above, the surface densification step of the presentinvention is conducted on one or more generally cylindrical surfaces (orsurface regions) of the pre-form. Thus, as will be apparent to personsskilled in the art, the invention allows the use of less expensive (andeasier to use) roller apparatus to achieve the desired densification.Moreover, by not initially forming (to the desired shapes) the surfacesto be densified, the invention makes it possible to achieve a uniformdensification over the entire area being densified. Furthermore, the useof a roller densification apparatus, in accordance with a preferredembodiment of the invention, results in an optimum surface finish anddimensional control, which is not possible with known shot peeningmethods. This, therefore, offers an important advantage over otherprocesses known in the art.

In the surface densification step, the surface regions being densifiedare provided with densities of at least 80% and up to 100% of thetheoretical maximum density. In a preferred embodiment, the densifiedsurface region has an approximate thickness of between 0.025 mm to 1 mm(i.e. 0.001 to 0.04 inches) below the surface. The core density of thepre-form is not significantly altered during the surface densificationstep and, therefore, the core of the pre-form remains the same as thatresulting from the compaction step.

The article may subsequently be annealed, at temperatures between 800°and 1100° C. in a protective atmosphere or vacuum, for the purpose ofdeveloping proper metallurgical bonding, re-crystallizing the densifiedsurface material and obtaining an advantageous microstructure forforming or contact fatigue durability.

The surface region being densified by this step may comprise either orboth of the inner and outer regions of the pre-formed article. Thisaspect is described, for example, in U.S. Pat. Nos. 5,540,883 and5,972,132 (the entire contents of which are incorporated herein byreference)

Forming of Article

The selectively densified article is then subjected to a formingoperation to achieve the desired final density, shape and dimensionalrequirements. The forming step is preferably carried out in a closed dieand at ambient temperatures, although, if required, elevatedtemperatures may also be used. The final density is obtained and closelycontrolled by the movement of the sintered material during forming andthe dimensions are controlled by the rigid die set. Such dies arecommonly known in the art. Where the final dimensions are not criticalto component functionality, complete filling of the die cavity may notbe required. The forming operation is alternatively referred to in theart as, inter alia, sizing, coining, repressing, forging or re-striking.These processes will be known to persons skilled in the art. All of theabove mentioned processes involve the application of pressure to asintered compact enclosed within a rigid die cavity. Conventional rigiddies as used in regular sizing/coining/repressing/restriking presses maybe used in the present invention to achieve the final surfaceconfiguration and higher density of the final article with precisecontrol. Forming is accomplished by the selection of the composition ofthe sintered article, by the selection of appropriate sinteringtemperature and furnace profile, by the selection of pressure used inthe forming operation, and the selection of the forming tool to providethe necessary clearance between the tools and the sintered article formovement of the sintered compact to the final shape. The required choiceof these parameters will be known to persons skilled in the art. Afterforming, the article will have a final core density of between 90% and98% of the theoretical maximum, and the densified surfaces will haveassumed the final configuration with overall radial dimensions of thecontoured form, differing by 0.1 to 10% as compared to the diameter ofthe surface densified region of the pre-form. Further, the final articlewill normally have a length dimension that is approximately between 5 to30% less than the same dimension measured on the sintered and surfacedensified pre-form.

Generally, as mentioned above, the article resulting from the surfacedensification step will have the approximate but not final shape of thedesired article. As such, the surface densified article will typicallynot occupy the entire volume of the forming die. FIGS. 1 to 4 illustratea die having a punch or ram with walls 12 and 14 and an outer die wall16. Typically, the outer wall 16 is stationary while the punch walls 12and 14 are designed to move towards and away from each other. It will beunderstood that, in some systems, one of punch walls 12 or 14 may alsoremain stationary. The combination of these elements (12, 14, and 16)form a die cavity 20 into which a sintered pre-form 22 is inserted.According to the present invention, the surface of the pre-form willhave a densified layer 23, having a generally cylindrical geometry aredescribed above. As also described above, the die cavity is of the shapeof the desired final product. As shown in FIGS. 1 and 2, the pre-form isdimensioned to be smaller than the die cavity, thereby leaving aclearance 24 between the pre-form 22 and the outer walls 16 and 18. Asthe punch walls 12 and 14 are moved towards each other, the pre-form iscompressed and radially expanded until is fills, and assumes the shapeof, the die cavity 20. The final punch position is illustrated in FIGS.3 and 4, which also show the final formed article 26 with the densifiedsurface 28 after having assumed the shape of the die walls 16 and 18.

FIGS. 1 to 4 only illustrate a die for the forming operation. It will beunderstood by persons skilled in the art that the actual shape andconfiguration of the die will depend upon the specific article beingformed. For example, the die can include core rods, moveable outer wallsor other configurations necessary to achieve the final article shape. Itwill be noted that FIGS. 1 to 4 serve to illustrate a forming operationconducted on the outer surface of a sintered pre-form. However, as willbe understood by persons skilled in the art, the forming die can be usedto provide the article with a desired outer and/or inner shape also asknown in the art. Similarly, the forming operation can be used to formmultilevel parts, such as an over-running clutch or other such articlesas known in the art.

Heat Treatment of Formed Article

Subsequent to forming, the article may optionally be annealed, attemperatures between 800 and 1300° C. in a protective atmosphere orvacuum and with suitable cooling in order to obtain proper metallurgicalbonding and to fully develop the desired mechanical properties.

The final article is usually required to have high wear and fatigueresistance. For this reason, heat treatment such as carburizing,quenching and tempering, etc., may be applied to an article made from ablend with 0.4% or less carbon, while through hardening or inductionhardening and tempering, etc., can be performed on a part containinggreater than 0.4% carbon. A prior through hardening treatment, eitherapplied as a forced cooling, or quenching following annealing or as aseparate heat treatment operation may be applied to increase the coreyield strength. Both methods produce an article with a hardened surfacecase and a hard core that is resistant to wear and exhibits superiorfatigue performance. Various other heat treatment methods will be knownto persons skilled in the art.

The invention described herein relates to the surface densification of aPM article while it still has a simple cylindrical geometry (i.e. priorto final forming of the article) and utilizes the ductility of thepre-formed material to impart the final contoured shape to the densifiedsurface. As will be appreciated by persons skilled in the art, theinvention provides an improvement over previously known methods, whichrequire surface densification of the final formed article. As will beunderstood by persons skilled in the art, one of the key advantages ofthe present invention lies in its ability to provide an improved,efficient process for producing a PM article having a complex shape andwith specific surface densification. As indicated above, it is oftenvery difficult or impossible to selectively densify complex surfacessince the densification apparatus known in the art can only accommodatesimple (i.e. cylindrical bearings) or specific (i.e. gear teeth) shapes,or require multiple passes through different dies. As also describedabove, the prior art methods require complex densification methods andapparatus to achieve surface densification of irregularly shapedobjects. Articles made according to the present invention may includeany powder metal article such as gears, bearings, cams etc. as will beapparent to persons skilled in the art.

The invention will now be described with reference to certain specificexamples. It will be understood that the following examples are meantonly to illustrate the invention and are not intended to limit the scopeof the invention in any way.

EXAMPLE 1 Carbon Manganese Molybdenum Outer Race

Iron powder, lubricant, graphite, ferromanganese and ferromolybdenumwere blended to achieve a sintered composition of approximately 0.2%carbon, 0.9% manganese and 0.5% molybdenum. The powder was formed intorings, which were compacted to a density of 6.5 g/cc (approximately 83%of the theoretical maximum) with a pressure of about 350 MPa. Thecompacted rings were sintered at 1280° C. for 20 minutes. Anitrogen/hydrogen atmosphere was maintained throughout the cycle.

The bore of each sintered ring was surface densified by the methoddescribed in U.S. Pat. No. 6,110,419 (incorporated herein by reference)thereby achieving a local surface density in excess of 99% of thetheoretical maximum density, while the core density remained at 6.5g/cc. This density profile is illustrated in FIG. 5. The bore surfacedensified rings were formed in a closed die with a core rod having thegeometry of the final cam form. The material exhibited remarkableductility and densification. At forming pressures of 700 to 1050 MPa,core densities of 7.30 To 7.55 g/cc were obtained as illustrated in FIG.6. Axial closures over the same pressure range were 15 to 18% of thesintered length (as illustrated in FIG. 7), and radial movement of up to4% of the sintered outer radius was achieved (as illustrated in FIG. 8).Following the forming step, the densified bore layer was intact (FIG.5), the core density was increased as indicated above, the surface hadsubstantially assumed the final cam shape and the active cam formexhibited both excellent surface finish and dimensional stability.

EXAMPLE 2 Carbon Molybdenum Inner Race

A blend with a sintered composition of 0.6% carbon and 0.9% molybdenumwas prepared by combining iron powder, ferromolybdenum, graphite, andlubricant. Rings of the powder blend were compacted to 85% oftheoretical maximum density with pressure of approximately 520 MPa. Therings were sintered in a nitrogen/hydrogen atmosphere for 20 minutes at1280° C. followed by an isothermal hold (as described in U.S. Pat. No.5,997,805, incorporated herein by reference) resulting in a malleablesintered article. The cylindrical outer surface of the sintered ringswas selectively densified using the roller burnishing method (asdescribed in U.S. Pat. No. 5,540,883, incorporated herein by reference)to achieve a surface density of greater than 99% theoretical maximum asillustrated in FIG. 9. As in Example 1, the material exhibitedremarkable ductility and through-densification. Core densities of 7.20to 7.45 g/cc were achieved at forming pressures of 750 to 1050 MPa (asillustrated in FIG. 10) resulting in axial closures of 12 to 16% of thesintered length (as illustrated in FIG. 11). Radial movement of up to 4%of the sintered inner radius was obtained as shown in FIG. 12. As inExample 1, after the forming step, the densified bore layer was intact(FIG. 9), the core density was increased as said, the surface hadsubstantially assumed the final cam shape and the active cam formexhibited both excellent surface finish and dimensional stability.

Although the invention has been described with reference to certainspecific embodiments, various modifications thereof will be apparent tothose skilled in the art without departing from the scope of theinvention as outlined in the claims appended hereto. The disclosures ofall references recited above are incorporated herein in their entirety.

1. A method for producing a powder metal article having a threedimensional shape and having at least one densified surface region, saidmethod comprising: a) providing a blend of powdered metals; b)compacting the blend to form a pre-form having a general shape of thearticle, said preform being generally cylindrically shaped at a regioncorresponding to said at least one densified surface region; c)sintering said pre-form; d) densifying said at least one surface regionof said pre-form; and, e) forming said pre-form to a desired finaldensity and into a desired three dimensional shape of said article. 2.The method of claim 1 wherein said step of densifying comprises coldrolling said at least one surface.
 3. The method of claim 1 furthercomprising: f) subjecting said article to annealing.
 4. The method ofclaim 1 further comprising: f) subjecting said article to heattreatment.
 5. The method of claim 1 wherein said powder is compacted instep (b) to a density of between 70% and 90% of the theoretical maximumdensity.
 6. The method of claim 1 wherein step (d) comprises densifyingsaid at least one surface region to a density of at least 80% of thetheoretical maximum density.
 7. The method of claim 1 wherein step (d)comprises densifying said at least one surface region to a density ofbetween 95% and 100% of the theoretical maximum density.
 8. The methodof claim 1 wherein said forming step includes compressing said pre-formto a core density of at least 90% of the theoretical maximum density. 9.The method of claim 1 wherein said forming step includes compressingsaid pre-form to a core density of between 90% and 98% of thetheoretical maximum density.
 10. The method of claim 1 wherein said atleast one surface region has a thickness of between 0.001 and 0.04inches after densification.
 11. The method of claim 1 wherein saidpowder metal blend comprises compressible iron powder, at least oneferro alloy, a lubricant, and carbon in the form of graphite.
 12. Themethod claim 11 wherein said ferro alloy comprises an alloy of iron witha metal chosen from the group consisting of: chromium, copper,manganese, molybdenum, nickel, niobium, vanadium, and combinationsthereof.
 13. The method of claim 12 wherein said ferro alloy is chosenfrom the group consisting of: ferro manganese, ferro molybdenum, andferro chromium.
 14. The method of claim 1 wherein said powder blendcomprises: elemental or substantially pure powder blends; fullypre-alloyed powder blends; partially pre-alloyed powder blends; or,powder blends containing ferro alloys.
 15. The method of claim 1 furtherincluding an isothermal treatment step following said sintering step(c).
 16. The method of claim 1 further including an isothermal treatmentstep, wherein said isothermal treatment is included in a cooling phaseof said sintering step (c).
 17. A method for producing a powder metalarticle having a three dimensional shape and having at least onedensified surface region, said method comprising: a) providing a blendof powdered metals; b) compacting the blend to form a pre-form having ageneral shape of the article, said pre-form having a density of between70% to 90% of the theoretical maximum density and being generallycylindrically shaped at a region corresponding to said at least onedensified surface region; c) sintering said pre-form; d) densifying saidat least one surface region of said pre-form to at least 80% of thetheoretical maximum density; and, e) forming said pre-form to a desiredfinal density and into a desired three dimensional shape of saidarticle.
 18. The method of claim 17 further including an isothermaltreatment step following said sintering step (c).
 19. The method ofclaim 17 further including an isothermal treatment step, wherein saidisothermal treatment is included in a cooling phase of said sinteringstep (c).
 20. A powder metal pre-form having a general shape of adesired article, said pre-form having a density of between 70% to 90% ofthe theoretical maximum density and being generally cylindrically shapedat least one surface region.
 21. A powder metal article formed from thepre-form of claim 20.